The preparation of cyclic materials is receiving more attention. A recent study involving the synthesis of cyclic oligomeric carbonates and their polymerization to high molecular weight polycarbonates is noteworthy: D. J. Brunelle, et al., ACS, Polymer Preprints, Vol. 30, No. 2, 1989, Miami Beach, Fla. Because the cyclic oligomeric products have relatively low molecular weights and viscosities, they are very suitable for molding applications and other reactive processing techniques. The advantages of "cyclics" technology in the preparation of thermoplastic composites is described in Modern Plastics, Nov. 1989, p. 10.
Another class of cyclic materials which has received considerable attention for over twenty years is the large ring cyclic compounds. In 1967, Pederson described his pioneering work on the synthesis of macrocyclic polyethers; Pedersen, C. J., Journal of the American Chemical Society, Vol. 89, p. 2495 and p. 7017, 1967. These cyclic crown ethers (I) were capable of forming surprisingly stable complexes with alkali and alkaline earth metal cations. The preparations of these "simple" cyclic products was followed by the preparation of other cyclic products having sulfur or nitrogen as donor atoms (II). Monocyclics were succeeded by bicyclic compounds (III) called cryptands. ##STR2## The scientific and practical interest in complexing cations by crown compounds is increasing--over 250 original papers dealing with crown ethers appeared in 1980 alone.
The unique ability of crown ethers to form stable complexes with various cations has been used in such processes as metal separation, transfer catalysts, increasing the activity of catalysts, solubilizing sodium and potassium carbonate in apolar solvents such as benzene, and catalysis of isocyanates to isocyanurates, see U.S. Pat. No. 4,487,928.
One of the main drawbacks in the use of crown compounds in industrial synthetic work is their cost due to the tedious methods needed for their preparation. High dilution techniques utilizing the "template" route is still the best way. Such routes not only use expensive reagents, but the yields are low and low volume production adds to the costs. ##STR3##